Apparatus for polishing thin, flat semiconductor wafers are well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semiconductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head, a wafer unload station, or a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is “planarized” or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in deionized water.
A schematic of a typical CMP apparatus is shown in FIGS. 1A and 1B. The apparatus 20 for chemical mechanical polishing consists of a rotating wafer holder 14 that holds the wafer 10, the appropriate slurry 24, and a polishing pad 12 which is normally mounted to a rotating table 26 by adhesive means. The polishing pad 12 is applied to the wafer surface 22 at a specific pressure. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films.
CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing, a metal oxide may be formed and removed separately.
A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off-center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is also rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel.
In a CMP head, large variations in the removal rate, or polishing rate, across the whole wafer area are frequently observed. A thickness variation across the wafer is therefore produced as a major cause for wafer non-uniformity. In the improved CMP head design, even though a pneumatic system for forcing the wafer surface onto a polishing pad is used, the system cannot selectively apply different pressures at different locations on the surface of the wafer. This effect is shown in FIG. 1C, i.e. in a profilometer trace obtained on an 8-inch wafer. The thickness difference between the highest point and the lowest point on the wafer is almost 2,000 angstroms, resulting in a standard deviation of 472 angstroms, or 6.26%. The curve shown in FIG. 1C is plotted with the removal rates in the vertical axis and the distance from the center of the wafer in the horizontal axis. It is seen that the removal rates obtained at the edge portions of the wafer are substantially higher than the removal rates at or near the center of the wafer. The thickness uniformity on the resulting wafer after the CMP process is poor.
The polishing pad 12 is a consumable item used in a semiconductor wafer fabrication process. Under normal wafer fabrication conditions, the polishing pad is replaced after about 12 hours of usage. Polishing pads may be hard, incompressible pads or soft pads. For oxide polishing, hard and stiffer pads are generally used to achieve planarity. Softer pads are generally used in other polishing processes to achieve improved uniformity and smooth surfaces. The hard pads and the soft pads may also be combined in an arrangement of stacked pads for customized applications.
A problem frequently encountered in the use of polishing pads in oxide planarization is the rapid deterioration in oxide polishing rates with successive wafers. The cause for the deterioration is known as “pad glazing”, wherein the surface of a polishing pad becomes smooth such that slurry is no longer held in between the fibers of the pad. This physical phenomenon on the pad surface is not caused by any chemical reactions between the pad and the slurry.
To remedy the pad glazing effect, numerous techniques of pad conditioning or scrubbing have been proposed to regenerate and restore the pad surface and thereby restore the polishing rates of the pad. The pad conditioning techniques include the use of silicon carbide particles, diamond emery paper, blade or knife for scraping or scoring the polishing pad surface. The goal of the conditioning process is to remove polishing debris from the pad surface and re-open pores in the pad by forming micro-scratches in the surface of the pad for improved pad lifetime. The pad conditioning process can be carried out either during a polishing process, i.e. known as concurrent conditioning, or after a polishing process.
While the pad conditioning process improves the consistency and lifetime of a polishing pad, a conventional conditioning disk is frequently not effective in conditioning a pad surface after repeated usage. A conventional conditioning disk for use in pad conditioning is shown in FIGS. 2A, 2B and 2C.
Referring next to FIG. 2A, a conventional CMP apparatus 50 includes a conditioning head 52, a polishing pad 56, and a slurry delivery arm 54 positioned over the polishing pad. The conditioning head 52 includes a conditioning disk 68 which is mounted on a conditioning arm 58 which is extended over the top of the polishing pad 56 for making a sweeping motion across the entire surface of the polishing pad 56. The slurry delivery arm 54 is equipped with slurry dispensing nozzles 62 which are used for dispensing a slurry solution on the top surface 60 of the polishing pad 56. Surface grooves 64 are further provided in the top surface 60 to facilitate even distribution of the slurry solution and to help entrapping undesirable particles that are generated by coagulated slurry solution or any other foreign particles which have fallen on top of the polishing pad 56 during a polishing process. The surface grooves 64, while serving an important function of distributing the slurry, also presents a processing problem when the pad surface 60 gradually wears out after prolonged use.
The conventional conditioning disk 68 may be of several different types. A conventional brazed grid-type conditioning disk is formed by embedding or encapsulating diamond particles in random spacings with each other in the surface of a stainless steel substrate. A conventional diamond grid-type conditioning disk is formed by embedding cut diamonds at regular spacings in a nickel film coated onto the surface of a stainless steel substrate. The diamonds are typically coated with a diamond-like carbon (DLC) layer.
Referring next to FIGS. 2B and 2C, the CMP apparatus 50 typically further includes a polishing head 70 which is mounted on a rotatable shaft 72 above the top surface 60 of the polishing pad 56. The polishing head 70 holds and rotates a wafer (not shown) against the top surface 60 of the polishing pad 56 to polish the wafer. Before production wafers are polished using the CMP apparatus 50, time must be allotted to warm the polishing pad 56 and facilitate flow of polishing slurry from a slurry container (not shown) to the slurry delivery arm 54. This enhances polishing uniformity among successive wafers polished on the apparatus 50.
Conventional techniques for warming the polishing pad 56 preparatory to polishing of production wafers thereon include successive mounting of typically 3-4 dummy wafers 74 on the polishing head 70 and rotation of each dummy wafer 74 against the top surface 60 of the polishing pad 56, as shown in FIG. 2C. After use, the dummy wafers 74 may be recycled, and eventually, discarded. While this technique is useful in pre-conditioning the polishing pad 56, the cost of the dummy wafers 74 is inordinately high, and thus, best avoided. Accordingly, a new and improved apparatus and method is needed for the pre-conditioning of a polishing pad in a CMP apparatus.
It is an object of the present invention to provide a new and improved apparatus which is suitable for the pre-conditioning of a polishing pad on a CMP apparatus.
Another object of the present invention is to provide a new and improved apparatus which is suitable for rotary-type CMP apparatus.
Still another object of the present invention is to provide a new and improved CMP pad pre-conditioning apparatus which is economical in operation.
Yet another object of the present invention is to provide a new and improved CMP pad pre-conditioning apparatus which utilizes an ingot to pre-condition a polishing pad prior to the polishing of production semiconductor wafers.
A still further object of the present invention is to provide a new and improved method for pre-conditioning a CMP polishing pad.
Yet another object of the present invention is to provide a new and improved method for pre-conditioning a CMP polishing pad, which method is economical and may be used without dummy wafers.
Another object of the present invention is to provide a new and improved apparatus and method which saves time in the pre-conditioning of a polishing pad on a CMP apparatus.
Still another object of the present invention is to provide a new and improved apparatus and method which may be adapted to pre-condition a variety of substrates including but not limited to polishing pads.